The present invention relates to solid-state laser devices and to integrating spheres, and more particularly to reflectors and heat insulators for solid-state laser devices, an arrangement for providing optimized excitation light to the solid-state laser medium, and integrating spheres for use in optical measuring systems.
On account of their compactness and ease of use, solid-state lasers as typified by the YAG laser have been used extensively, especially in the laser machining field. Recently, solid-state lasers have also found wide application in the measurement and medical fields.
Challenges facing the designers of solid-state lasers are raising the oscillation efficiency of the lasers and eliminating undesirable effect caused by heat build-up in the laser medium. In order to solve these problems, the following requirements must be satisfied: (a) the light emitted by the excitation light source must be efficiently introduced in the laser medium, and (b) the excitation light must have wavelength distribution suitable for laser oscillation.
In a typical solid-state laser device, a laser medium is in the shape of a rod or a slab (plate), and an electric discharge tube, e.g., a krypton flash lamp disposed parallel to the laser medium, is used for exciting the laser medium to emit light for oscillation. For laser efficiency, the manner in which excitation light is injected into the laser medium is important. For light injection, reflectors made from various highly reflective materials have been used. Most common among these are gold-plated copper or brass reflectors. Also known are silver-plated reflectors and ceramic reflectors.
A similar need for high reflectance arises with so-called integrating spheres as used in optical measurement systems. If a conventional optical detector is used to measure light energy, for example, the measured value may depend on the size of a light-receiving surface and on the intensity distribution of incident light across the light-receiving surface. Thus, different beams with the same energy may produce different measurement readings. For more consistent readings, an integrating sphere in which injected light is repeatedly and diffusely reflected at its inner surface may be utilized so that the intensity distribution of the light is made uniform. For high reflectance of the inner surface of the integrating sphere, white coatings having MgO or BaSO.sub.4 as their main component are typically used.
Gold-plated reflectors as mentioned above have desirably high reflectance, especially for light of wavelength near 0.8 .mu.m, which contributes greatly to the oscillation of YAG lasers, with little decrease in reflectance of the plated surface due to contamination and deterioration. However, since there is considerable absorption loss for light in the 0.5 to 0.6 .mu.m absorption band of Nd:YAG crystals, and the reflectance of gold-plated surfaces falls off at wavelengths below 0.6 .mu.m, excitation efficiency of gold-plated reflectors remains limited. Furthermore, because of large absorption in the 0.5 to 0.6 .mu.m wavelength band, gold-plated reflectors are not suitable for solid-state laser devices having laser crystals doped with Cr.sup.3+, such as Cr:Nd:GSGG and Cr:BeAl.sub.2 O.sub.2 (alexandrite), as the laser medium.
As reflectors for short-wavelength light, silver-plated reflectors are problematic because silver forms sulfides having decreased reflectance. This difficulty can be overcome, e.g., by coating the silver surface with a protective film of SiO.sub.2 for example, or by silver-plating the back surface of a glass plate so that the silver surface does not come into direct contact with cooling water. In high-power lasers, however, silver-plate surfaces are less suitable because of the likelihood of damage caused by heat produced by the excitation light.
Ceramic materials have also received recent attention as reflector materials for short-wavelength light, but the reflectance of such materials may not always be sufficient. Reflectance may be lower still with reflectors in which so-called free-cutting ceramics are used, which are made by dispersing ceramic particles in a glass matrix.
The above-mentioned problems relating to reflectors for laser devices similarly apply to integrating spheres. Although, for weak light, the above-mentioned coatings containing MgO, BaSO.sub.4 or the like are well suited for integrating spheres, at higher powers these coatings' resistance to light becomes unsatisfactory.
In order to achieve improved oscillation with a slab-shaped crystal laser medium, not only the reflector but also the heat insulators for the crystal must be made of a material which does not absorb light. In order to form a one-dimensional thermal gradient, the laser medium of a slab laser has a structure in which a pair of side surfaces of the slab are cooled, between which side surface the laser light propagates along a zigzag path due to repeated total reflection, and the remaining two side surfaces are provided with heat insulators to inhibit heat flow toward them. Since these heat insulators are positioned in the vicinity of the laser medium, their light absorption may cause not only reduction in the efficiency of the laser but also heating of the laser medium due to heat build-up in the heat insulators. Therefore, heat insulators used in a slab laser must have high reflectance or transmittance to the laser light and the excitation light, in addition to having low thermal conductivity. In the past, glass, ceramics and the like have been used for such heat insulators.
Another method of improving oscillation efficiency is to convert the wavelength distribution of the excitation light to the distribution suitable for the excitation of the laser medium. This is based on the fact that only a small range of the spectrum of light emitted by the flash lamp is suitable for laser oscillation, which is a major reason for the low efficiency of a lamp-excited solid state laser.
In the above wavelength-conversion approach, the wavelength of the excitation light is converted by using a substance that emits fluorescent light having a wavelength suitable for the laser oscillation. One well-known method, as disclosed in Japanese unexamined patent publication (Kokai) No. S61-23374, is to use a piece of glass doped with samarium (Sm) or a piece of glass doped with cerium (Ce) as a filter for excitation light in a solid state laser utilizing a laser medium doped with Nd.sup.3+ as an activator. The filter absorbs light in the ultraviolet region and emits fluorescent light having a wavelength .lambda. of about 0.6 .mu.m. Since Nd.sup.3+ has one of its absorption peaks in this wavelength region, the fluorescent light is efficiently absorbed to improve oscillation efficiency.
A similar method is disclosed in Japanese unexamined patent publication (Kokai) No. H2-123776, which discloses an invention of the present inventor, in which an AlGaAs semiconductor is used to convert the wavelength of the excitation light. Since the band gap of an AlGaAs semiconductor corresponds to photoenergy of around wavelength .lambda.=810 nm, absorbed excitation light with a wavelength shorter than 810 nm excites carriers above the conduction band with subsequent recombination that results in emission of light of wavelength .lambda. around .lambda.=810 nm as a fluorescent light. Since light with 810 nm wavelength is most suitable for exciting Nd.sup.3+, highly efficient laser oscillation is achieved for laser medium doped with Nd.sup.3+.
In both the method utilizing glass filters doped with Sm or Ce and the method utilizing AlGaAs semiconductor filters, a part of unutilized energy is converted to thermal energy in such filters, which reduces the thermal load to the laser crystal. Therefore, the above-described filters are effective means for solving the problems attributable to the heat developed in a solid state laser crystal. However, it has been known that the method of providing an excitation light filter with glass being doped with Sm or Ce gives low fluorescent emission efficiency which only slightly improves the efficiency of a solid state laser. Converting the wavelength of excitation light with an AlGaAs semiconductor is a promising method, but it still has a problem in that the refractive index of AlGaAs is as high as 3 or more. If the refractive index is high, most of the excitation light is reflected to be only a small amount of incident light. In addition, the light which has been wavelength-converted therein is not easily emitted to the outside. Further, this material has a practical difficulty of being susceptible to oxidation.
In general, the absorption spectrum of Nd.sup.3+ doped in a solid state laser medium exhibits a localized distribution similar to a line spectrum, which poorly matches the spectral distribution of light from a lamp. This is a major cause of low efficiency of a Nd:YAG laser. With respect to improvement of the efficiency of a solid state laser utilizing Nd.sup.3+ as an activator, Koechner describes (W. Koechner, Solid State Laser Engineering, page 57, (3rd ed., Spinger-Verlag Corp., 1992)) a method utilizing a laser medium doped with multiple elements such as Gd.sub.3 Sc.sub.2 Ga.sub.3 O.sub.12 doped with Nd and Cr (Nd, Cr:GSGG). Unlike Nd.sup.3+, Cr.sup.3+ exhibits a absorption spectrum widely ranging from 400 to 650 nm. As a result, when a combination of Nd and Cr:GSGG is used, Nd.sup.3+ is excited by the so-called sensitization, wherein Cr.sup.3+ absorbs excitation light with wavelengths in a wide range and transfers its energy from the excited state of Cr.sup.3+ to that of Nd.sup.3+. This results in an improvement in efficiency compared with the case where only Nd.sup.3+ is used, because excitation light with a wider range of wavelengths is effectively used. In essence, this method changes the absorption spectrum of the laser medium instead of converting the wavelength of the excitation light.
A method based on a somewhat different idea is disclosed in Japanese examined patent publication (Kokoku) No. H5-66035, which discloses a solid state laser rod doped with multiple elements, such as Nd and Cr:GSGG, and a Nd:YAG rod arranged in series. The two laser rods are simultaneously excited by introducing excitation light from the side of the multi-element solid state laser rod. The multi-element solid state laser medium absorbs light with a wide range of wavelengths for oscillation, and light which has passed through the multi-element medium excites the Nd:YAG for oscillation. Since the light which has passed through the multi-element solid state laser has wavelength distribution suitable for the excitation of the Nd:YAG, highly efficient laser oscillation can be achieved as a whole.
Although the method utilizing a solid state laser medium doped with multiple elements such as Nd and Cr:GSGG provides an effective means for improving efficiency, reduction in the thermal load to the laser medium can not be expected because excessive thermal energy is produced in the crystal. GSGG or the like has a problem in that it is more susceptible to heat because its thermal conductivity is smaller than that of YAG.
In the method wherein a multi-element solid state laser rod doped with elements such as Nd and Cr:GSGG is placed in series with an Nd:YAG rod and are simultaneously excited by introducing excitation light from the side of the multi-element solid state laser rod, a problem arises in that laser light is oscillated with two different wavelengths. Although the oscillation of two wavelengths is acceptable in some applications, it results in many difficulties including the fact that the wide range of wavelengths increases the effect of chromatic aberration of the lens. This method is also much susceptible to heat, similar to the case in which Nd or Cr:GSGG is used alone.
In the context of heat insulators, glass, which has been used in slab lasers, absorbs neither excitation light nor laser light. However, glass poses problems in machining and in being insufficiently strong. Ceramics are relatively well suited as a heat insulator, but they have a problem of being heated by absorbing light in the region of short wavelengths. Further, both glass and ceramics lack sufficiently low thermal conductivity as a heat insulator.